Article Date: 7/1/2003

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Corneal Distortion from a Defective Hydrogel Lens
BY JOSEPH G. VEHIGE, OD

Hydrogel contact lenses are typically uniform in dimension. They represent hemispherical shells with a given central radius of curvature and, depending on the design, flatter peripheral curves which have a slightly greater radius of curvature than the central curve.

Hydrogel contact lenses manufactured in dry (non-hydrated) conditions are typically similar in shape and dimension, but they are proportionally smaller than the finished, hydrated lens. Specifically, lathe cut, spin cast and dry-molded lenses are miniature versions of the final contact lens. They will "grow" through expansion as the material hydrates to produce a lens that meets final target parameters.

Figure 1. Fundus reflex illumination of defective tinted lens on eye.

If a hydrogel lens does not expand uniformly, then it typically becomes distorted. Several forms of distortion can result (ovals, "tacos," fluted edges) based on which area of the lens is affected and whether the polymer in that area "over swells" or "under swells."

Effects of Lens Distortion

Figures 1 and 2 show a defective, opaque, tinted hydrogel lens that produced ripples and dimples across the optic zone, which were easily visible in fundus reflex illumination. I assumed that the distortion resulted from excessive lens material centrally that could not adopt a normal "parallel" alignment with central cornea. The excess central lens bulk covered the cornea in a non-uniform manner similar to a collapsed tent or deflated hot air balloon. However, because the hydrogel material had some structure, it produced negative pressure under the lens and resulted in rapid corneal distortion.

The lens surface under these conditions became dynamic, shifting and rearranging between blinks. Optically the lens did not produce a stable power, and image quality was poor. I observed parallel positive cylindrical elements, which resembled the surface of a Maddox rod.

Figure 2. This defective hydrogel lens caused optical zone distortion.

Approximately 15 minutes after I removed the contact lens, I observed subtle reversible corneal distortion through fluorescein pooling. The patient's spectacle acuity temporarily worsened.

This distorted hydrogel lens may have resulted from non-uniform expansion during hydration. It appears that either the peripheral zone (holding the opaque elements) did not fully expand (girdling) or the central clear zone had expanded excessively. The lens was otherwise round and appeared normal. I could not anticipate this form of lens distortion before lens application.

In this case, the unusual lens-induced mechanical forces showed that the corneal surface is quite malleable and, despite only brief contact, the cornea distorted enough to affect acuity. Perhaps the closest common analogy to this occurs when vigorous eye rubbing results in temporary "corneal shagreen," which you can see with fluorescein. In any event, and perhaps this is true for similar, short-duration mechanical distortion, the loss of acuity is transient and visible distortion rapidly reverses.

Dr. Vehige is the senior director of Consumer Eye Care Research and Development for Allergan, Inc. He is an experienced clinical study investigator and has published numerous scientific papers and posters.



Contact Lens Spectrum, Issue: July 2003